Global energy interconnection (GEI) is a globally interconnected strong and smart grid with UHV grid as the backbone, which will serve as a platform for extensive development, deployment and utilization of clean energy globally. The HWLT is a kind of three-phase AC transmission of which the line length is half wavelength, meaning 2500 km for 60 Hz or 3000 km for 50 Hz. As theoretically the charging power of HWLT is equal to the reactive power loss, the sending end can be considered electrically located at the receiving end. Thus no reactive compensation devices and intermediate substations are needed, and HWLT is more economic compared to other transmission modes when the transmission length is close to
half wavelength.
source: G. Samorodov, S. Kandakov, S. Ziberman, et al. “Technical and economic comparison between direct current and half-wavelength transmission systems for very long distances,” IET Generation, Transmission & Distribution, vol. 11, no. 11, pp. 2871-2878, 2017.
Measuring the voltage angle is important and therefore PMUs are the actual technology that can provide the desired results.
The theoretical situation corresponds to the case of surge impedance loading, which is not an economic operation. Also, this operating state cannot be maintained for long term so large voltage deviations are experienced when the transmitted power differs from SIL value. This is why many intermediate substations are required, which means additional reliability and exploitation problems.
The classical technique are both bulky and iterative in nature for transmission power flow measurement. The modern technology use PMUs. Please see the paper "Synchrophasor measurement method for overhead line parameters estimation in MV distribution networks" from https://ieeexplore.ieee.org/document/6842833/
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